Method of fabricating semiconductor device

ABSTRACT

A method of fabricating a semiconductor device having high output power and excellent long-term reliability by preventing thermal adverse influence exerted at the time of window structure formation is provided. The method comprises a 1st step of forming predetermined semiconductor layers  2  to  9  containing at least an active layer  4   b  consisting of a quantum well active layer on a semiconductor substrate  1 ; a 2nd step of forming a first dielectric film  10  on a first portion of the surface of the semiconductor layers  2  to  9 ; a 3rd step of forming a second dielectric film  12  made of the same material as that of the first dielectric film  10  and having a density lower than that of the first dielectric film  10  on a second portion of the surface of the semiconductor layers  2  to  9 ; and a 4th step of heat-treating a multilayer body containing the semiconductor layers  2  to  9 , the first dielectric film  10 , and the second dielectric film  12  to disorder the quantum well layer below the second dielectric film  12.

This is a continuation application of International application no.PCT/JP04/18695, filed Dec. 15, 2004, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of fabricating a semiconductordevice that includes portions to be partially disordered, such as awindow structure.

RELATED ART

Conventionally, there has been a problem that semiconductor laserdevices are susceptible to instantaneous deterioration by a catastrophicoptical damage (COD) and it has been a factor to prevent thesemiconductor laser from having a high output power. The COD is aphenomenon in which a cycle that: a recombination current caused bynon-radiative recombination flows through a light-emitting facet side ofan active layer of the semiconductor laser, which results in theincrease in temperature of the facet, and the increased temperatureleads to further shrinkage in the band gap energy and increase in thelight absorption, is created, which becomes a positive feedback to causemelting of the facet.

In order to prevent such COD, a so-called window structure, which hasportions in an activity layer near the light-emitting facet made of amaterial with a larger band gap energy than that at the central portionof the activity layer, is effective. The window structure has a largerband gap energy at the laser light emitting facet, whereby absorption ofthe laser light becomes small and the COD can be suppressed.

The window structure has been conventionally formed by an independentsemiconductor process. For example, portions where the windows are to beformed are removed by etching or the like, and thereafter, materialswith property corresponding to the window are embedded into theportions. On the other hand, formation of the window structure is alsoachieved by disordering (mixed crystallization) of the portions to bethe windows. When the active layer has a quantum well structure, theprocess of disordering is performed by ion implantation (Patent document1), introduction of impurities (Patent document 2), dielectric filmformation, and the like. All of them are processes to generate atomicvacancies in the semiconductor crystal and, by diffusion of atomicvacancies, to make the crystal structure of the quantum well activelayer irregular and disordered. The portions disordered in this wayexhibit properties different from those before implementing the processof disordering, for example, a different band gap energy, a differentrefractive index, and the like. Utilizing the above, the band gap energynear the facet of the semiconductor laser can be enlarged, and the CODcan be suppressed.

Among the above processes of disordering, the process utilizing adielectric film includes the steps that: by forming a dielectric film onthe surface of a semiconductor multilayer and heating, the constituentatoms in the semiconductor are made to diffuse into the dielectric film,which generates atomic vacancies are generated in the semiconductor, anddiffusion of the atomic vacancies lead to disordering of thesemiconductor crystal. As the dielectric film, SiO₂ has been usedconventionally (Patent document 3). The disordering method usingdielectric films is superior to the ion implantation method or the likeat the point that it introduces less defects into the crystal.

Patent document 1: Japanese Patent Application Laid-Open No. 10-200190

Patent document 2: Japanese Patent Application Laid-Open No. 2000-208870

Patent document 3: Japanese Patent Application Laid-Open No. 5-29714

However, when disordering is implemented by utilizing the dielectricfilm, a thermal treatment must be performed as mentioned above. Sincethe thermal treatment is performed on the entire semiconductor laserdevice, it may sometimes adversely affects portions not to bedisordered. For example, when the active layer is made of AlGaAs basedmaterial, there has been a problem that As atoms are desorbed from thesemiconductor surface corresponding to the active layer region not to bedisordered and the semiconductor surface is roughed. Therefore,favorable contact is not assured when an electrode is formed on acontact layer, which affects adversely the performance characteristicsof the semiconductor laser.

Also, desorption of As leaves pits (small holes) on the semiconductorsurface, and dislocation defects are generated by the pits. This isexplained using FIG. 12. FIG. 12 is a longitudinal section of asemiconductor laser device having conventional window structures in thedirection of a laser resonator, schematically showing events during thethermal treatment for disordering. In FIG. 12, the window 28 isconstituted by forming a SiO₂ disordering-enhancing film 42 on thesurface of the semiconductor laser at the upper part of the windowformation region 28 a, and by performing the thermal treatment fordisordering. In this process, As atoms are desorbed from thesemiconductor laser device surface in the region 28 b where the windowis not formed, and leave pits. The pits propagate, forming dislocationdefects 41, reach the active layer 4, and deteriorate the laserperformance or spoil the long-term reliability.

Adverse effects by the thermal treatment have not been restricted to theportions not to be disordered, as mentioned above. That is, also on theportion to be disordered, oxygen atoms from the SiO₂disordering-enhancing film 42 mingle into the semiconductor crystal tobecome crystal defects and have been the cause to spoil the long-termreliability of a semiconductor laser.

The present invention has been made in view of the above problems. Anobject of the present invention is to prevent the adverse effects ofthermal treatment when the window structure or the like is formed in asemiconductor laser device, and to enable fabricating a semiconductordevice of high output power and excellent long-term reliability.

SUMMARY OF THE INVENTION

The present invention is made in order to attain the above-mentionedobject, and relates to a method of fabricating a semiconductor devicethat includes disordered portions.

A method of fabricating a semiconductor device according to a firstembodiment of the invention comprising the steps of:

forming a predetermined semiconductor multilayer which includes at leastan active layer consisting of a quantum well active layer on asemiconductor substrate;

forming a first dielectric film on a first portion of the surface ofsaid semiconductor multilayer;

forming a second dielectric film made of the same material as said firstdielectric film and having a lower density than said first dielectricfilm on a second portion of the surface of said semiconductormultilayer;

performing thermal treatment of a multilayer body containing saidsemiconductor multilayer, said first dielectric film and said seconddielectric film, to disorder said quantum well layer under said seconddielectric film; and

cleaving said multilayer body at nearly the central part of said secondportion.

A method of fabricating a semiconductor device according to a secondembodiment of the invention comprising the steps of:

forming a predetermined semiconductor multilayer which includes at leasta quantum well active layer on a semiconductor substrate;

forming a first dielectric film on a first portion of the surface ofsaid semiconductor multilayer;

forming a second dielectric film made of the same material as said firstdielectric film and having a lower refractive index than said firstdielectric film on a second portion of the surface of said semiconductormultilayer;

performing thermal treatment of a multilayer body containing saidsemiconductor multilayer, said first dielectric film and said seconddielectric film, to disorder said quantum well layer under said seconddielectric film; and

cleaving said multilayer body at nearly the central part of said secondportion.

In the method of fabricating the semiconductor device according to athird embodiment of the invention, the refractive index of said firstdielectric film is larger than a predetermined value determineddepending on the film formation conditions of said first dielectric filmand said second dielectric film, and the refractive index of said seconddielectric film is smaller than said predetermined value.

A method of fabricating a semiconductor device according to a fourthembodiment of the invention comprising the steps of:

forming a predetermined semiconductor multilayer which includes at leasta quantum well active layer on a semiconductor substrate;

forming a first dielectric film containing silicon on a first portion ofthe surface of said semiconductor multilayer;

forming a second dielectric film made of the same material as said firstdielectric film and with a lower silicon composition ratio than saidfirst dielectric film on a second portion of the surface of saidsemiconductor multilayer;

performing thermal treatment of a multilayer body containing saidsemiconductor multilayer, said first dielectric film and said seconddielectric film, to disorder said quantum well layer under said seconddielectric film; and

cleaving said multilayer body at nearly the central part of said secondportion.

In the method of fabricating the semiconductor device according to afifth embodiment of the invention, the silicon composition ratio of saidfirst dielectric film is larger than the stoichiometric compositionratio of said dielectric film, and the silicon composition ratio of saidsecond dielectric film is smaller than the stoichiometric compositionratio of said dielectric film.

A method of fabricating a semiconductor device according to a sixthembodiment of the invention comprising the steps of:

forming a predetermined semiconductor multilayer which includes at leasta quantum well active layer on a semiconductor substrate;

forming a first dielectric film on a first portion of the surface ofsaid semiconductor multilayer;

forming a second dielectric film made of the same material as said firstdielectric film and having a higher hydrogen concentration in the filmthan said first dielectric film on the second portion of the surface ofsaid semiconductor multilayer;

performing thermal treatment of a multilayer body containing saidsemiconductor multilayer, said first dielectric film and said seconddielectric film, to disorder said quantum well layer under said seconddielectric film; and

cleaving said multilayer body at nearly the central part of said secondportion.

In the method of fabricating the semiconductor device according to aseventh embodiment of the invention, said first dielectric film and saidsecond dielectric film are silicon nitride films.

In the method of fabricating the semiconductor device according to aneighth embodiment of the invention, said first dielectric film is formedby the steps of:

arranging a heat source in a chamber on a path through which a firstprecursor of said first dielectric film passes to cause a decompositionreaction of said first precursor in the presence of said heat source,and

exposing said first portion of said semiconductor device in saidchamber; and

said second dielectric film is formed by the steps of:

arranging a heat source in said chamber on a path through which a secondprecursor of said second dielectric film passes to cause a decompositionreaction of said second precursor in the presence of said heat source,and

exposing said second portion of said semiconductor device in saidchamber.

In the method of fabricating the semiconductor device according to aninth embodiment of the invention, said first dielectric film is formedby the steps of:

arranging a heat source in a chamber on a path through which a firstprecursor of said first dielectric film passes to cause a decompositionreaction of said first precursor in the presence of said heat source,and

exposing said first portion of said semiconductor device in saidchamber; and

said second dielectric film is formed by the steps of:

arranging a heat source in said chamber on a path through which a secondprecursor of said second dielectric film passes to cause a decompositionreaction of said second precursor in the presence of said heat source,and

exposing said second portion of said semiconductor device in saidchamber; and

said first and second precursors are compounds containing nitrogen andsilicon, or mixtures of nitrogen compounds and silicon compounds.

In the method of fabricating the semiconductor device according to atenth embodiment of the invention, said first precursor and said secondprecursor contain silane and ammonia, and the silane content in saidfirst precursor is larger than the silane content in said secondprecursor.

In the method of fabricating the semiconductor device according to aneleventh embodiment of the invention, said first dielectric film andsecond dielectric film are respectively formed by the step of causingdecomposition reactions of said first precursor and said secondprecursor with catalytic CVD method.

In the method of fabricating the semiconductor device according to atwelfth embodiment of the invention, said predetermined semiconductormultiplayer is formed by the steps of:

forming an optical waveguide layer, at least on one side of said quantumwell layer in the layer forming direction, and

embedding a semiconductor layer with a conduction type opposite to theconduction type of said optical waveguide layer into said opticalwaveguide layer at the portion beneath said second portion.

In the method of fabricating the semiconductor device according to athirteenth embodiment of the invention, said predetermined semiconductormultiplayer is formed by the steps of:

forming optical waveguide layers having band gap energies larger thanthe band gap energy of said quantum well layer on both sides of saidquantum well layer in the layer forming direction, respectively;

forming cladding layers having band gap energies larger than the bandgap energies of said optical waveguide layers on both sides in the layerforming direction of the multilayer structure consisting of said quantumwell layer and said optical waveguide layers, respectively; and

forming career block layers having band gap energies larger than eachband gap energy of said optical waveguide layers between said quantumwell layer and said optical waveguide layers.

In the method of fabricating the semiconductor device according to afourteenth embodiment of the invention, said predetermined semiconductormultiplayer is formed by the step of forming either a single or amultiple quantum well structure.

The method of fabricating the semiconductor device according to thepresent invention comprises also the following other modes.

A protective film formation process of forming a first dielectric filmas a protective film on the surface of a semiconductor device at aportion corresponding at least to the portion not to be disordered; adisordering-enhancing film formation process of forming a seconddielectric film made of the same material as said first dielectric filmand having lower density than said first dielectric film, on the surfaceof the semiconductor device at the portion corresponding at least to theportion to be disordered, as a disordering-enhancing film; and adisordering process of disordering said portion to be disordered bythermal treatment.

A protective film formation process of forming a first dielectric filmas a protective film on the surface of a semiconductor device at aportion corresponding at least to the portion not to be disordered; adisordering-enhancing film formation process of forming a seconddielectric film made of the same material as said first dielectric filmand has lower refractive index than said first dielectric film, as adisordering-enhancing film, on the surface of the semiconductor deviceat a portion corresponding at least to the portion to be disordered; anda disordering process of disordering said portion to be disordered bythermal treatment. This mode may include that the refractive index ofsaid first dielectric film is larger than the predetermined valuedetermined dependent on the film formation conditions of said firstdielectric film and said second dielectric film; and the refractiveindex of said second dielectric film is smaller than said predeterminedvalue.

A protective film formation process of forming a first dielectric filmcontaining silicon as a protective film on the surface of asemiconductor device at a portion corresponding at least to the portionnot to be disordered; a disordering-enhancing film formation process offorming a second dielectric film made of the same material as said firstdielectric film and having lower Si composition ratio than said firstdielectric film as a disordering-enhancing film on the surface of thesemiconductor device at the portion corresponding at least to theportion to be disordered; and a disordering process of disordering saidportion to be disordered by thermal treatment. This mode may containthat the Si composition ratio of said first dielectric film is largerthan the stoichiometric composition ratio of said dielectric film, andthe Si composition ratio of said second dielectric film is smaller thanthe stoichiometric composition ratio.

A protective film formation process of forming a first dielectric filmas a protective film on the surface of a semiconductor device at aportion corresponding at least to the portion not to be disordered; adisordering-enhancing film formation process of forming a seconddielectric film made of the same material as said first dielectric filmand a has higher hydrogen concentration in the film than said firstdielectric film as a disordering-enhancing film on the surface of thesemiconductor device at a portion corresponding at least to the portionto be disordered; and a disordering process of disordering said portionto be disordered by thermal treatment.

Said first dielectric film and said second dielectric film are siliconnitride films.

Said protective film formation process is performed by arranging a heatsource on a path through which a first precursor of the first dielectricfilm to be formed passes, to cause a decomposition reaction of saidfirst precursor in the presence of said heat source, and exposing atleast a portion of the surface of said semiconductor devicecorresponding to the portion not to be disordered to the atmosphereremained after said decomposition reaction; and saiddisordering-enhancing film formation process is performed by arranging aheat source on a path through which a second precursor of said seconddielectric film to be formed passes, to cause a decomposition reactionof said second precursor in the presence of said heat source, andexposing at least a portion of the surface of said semiconductor devicecorresponding to the portion to be disordered to the atmosphere remainedafter said decomposition reaction.

Said protective film formation process is performed by arranging a heatsource on a path through which a first precursor of the first dielectricfilm to be formed passes, to cause a decomposition reaction of saidfirst precursor in the presence of said heat source, and exposing atleast a portion of the surface of said semiconductor devicecorresponding to the portion not to be disordered to the atmosphereremained after said decomposition reaction; and saiddisordering-enhancing film formation process is performed by arranging aheat source on a path through which a second precursor of said seconddielectric film to be formed passes, to cause a decomposition reactionof said second precursor in the presence of said heat source, andexposing at least a portion of the surface of said semiconductor devicecorresponding to the portion to be disordered to the atmosphere remainedafter said decomposition reaction; and said first and second precursorsare compounds containing nitrogen and silicon, or mixtures of nitrogencompounds and silicon compounds.

Said first precursor and said second precursor contain silane (SiH₄) andammonia (NH₃), and the silane content in said first precursor is largerthan the silane content in said second precursor. Thereby, the firstdielectric film with a large silicon composition ratio and high density,and the second dielectric film with a small silicon composition ratioand low density, are formed.

Said protective film formation process and said disordering-enhancingfilm formation process are performed by utilizing catalytic chemicalvapor deposition (CVD) method.

Said semiconductor device has said disordered portion constituting, atleast in the vicinity of one of the facets in the resonance direction, awindow structure where laser light is not absorbed, and the portion notdisordered constituting an active layer with the quantum well structure.In this mode said semiconductor device may have current a non-injectionregion which block current injection into said disordered portion. It ispreferable that the length Ln of said current non-injection regionmeasured from said facet of said semiconductor device is in the range ofLw≦Ln≦Lw+10 μm, setting the length of said disordered portion measuredfrom said facet of the semiconductor device to be Lw. Moreover, it ispreferable that said current non-injection region is embedded in saidsemiconductor device and is a semiconductor layer having the conductiontype opposite to that of the surrounding semiconductor layer.

Said semiconductor device is provided with: n-type and p-type opticalwaveguide layers having a band gap energy larger than the band gapenergy of a said active layer on both sides of the active layer in thelayer forming direction, respectively; n-type and p-type cladding layershaving a band gap energy larger than the band gap energy of said opticalwaveguide layers, in a way to sandwich said active layer and saidoptical waveguide layer from both sides in the layer forming direction,respectively, and carrier block layers having a band gap energy largerthan each band gap energy of said active layer and said opticalwaveguide layer between said active layer and said optical waveguidelayers.

In the present invention, it is expected that when a dielectric filmwith high compactness and high density is formed on a semiconductorcrystal, its activity to absorb Ga atoms may be small, but when adielectric film with small density is formed on the semiconductorcrystal, its activity to absorb Ga atoms may be large. Thus, atomicvacancies are easily formed in the portion of the semiconductor crystalabove which a dielectric film with small density was formed, and atomicvacancies are rarely formed in the portion above which a dielectric filmwith large density was formed, and thus, upon thermal treatment,disordering of the multiple quantum well takes place under thedielectric film with small density, but does not under the dielectricfilm with large density. That is, upon disordering, a dielectric filmwith large density will function as a protective film, and a dielectricfilm with small density as an enhancing film.

On the other hand, it is known that magnitudes of physical properties ofdielectric films including densities can be judged by the magnitudes ofrefractive indices. The present inventor has found that whether any ofthe first and second dielectric films functions as a protective film oras a disordering-enhancing film, respectively, is possible to be judgedby focusing attention on the refractive indices of dielectric films,especially on the basis of predetermined values determined depending onfilm formation conditions, including film formation temperature andpressure, and film formation apparatus. That is, if the refractive indexof the formed first dielectric film is larger than a predetermined valueand the refractive index of the second dielectric film is smaller thansaid predetermined value, the first dielectric film can function as aprotective film and the second dielectric film as adisordering-enhancing film in the disordering thermal treatmentperformed afterwards.

In the present invention, the magnitude of the density of a dielectricfilm can also be distinguished by the magnitude of the composition ratioof Si in it. That is, focusing attention on the composition ratios of Siin the first and second dielectric films, if Si composition of theformed first dielectric film is larger than the stoichiometriccomposition ratio of said dielectric film and Si composition of thesecond dielectric film is smaller than the stoichiometric compositionratio of said dielectric film, the first dielectric film can function asa protective film and the second dielectric film as adisordering-enhancing film in the disordering thermal treatmentperformed afterwards.

Further, focusing attention on the quantity of the hydrogen contained inthe first and second dielectric films, if the quantity of hydrogen inthe first dielectric film is smaller than the quantity of hydrogen inthe second dielectric film, the first dielectric film can work as aprotective film and the second dielectric film as adisordering-enhancing film.

As for two kinds of dielectric films distinguished by each ofabove-mentioned standards, the first dielectric film has a high densityand its activity to absorb Ga atoms is small when it is formed on asemiconductor crystal, while the second dielectric film has a lowdensity and its activity to absorb Ga atoms is large when it is formedon a semiconductor crystal. For this reason, atomic vacancies are easilyformed in the portion of the semiconductor crystal under the seconddielectric film, and the atomic vacancy is hardly formed in the portionunder the first dielectric film. And thus, upon thermal treatment,disordering of a multiple quantum well takes place in the portion underthe second dielectric film, but does not take place in the portion underthe first dielectric film. That is, upon disordering treatment, thefirst dielectric film will function as a protective film, and the seconddielectric film will function as an enhancing film.

Said semiconductor device is, for example, a semiconductor laser deviceor the like, which has said disordered portion constituting a windowstructure which does not absorb laser light located in the vicinity ofat least one of the facets in the resonance direction, and thenon-disordered portion constituting an active layer of a quantum wellstructure.

Further, said semiconductor laser device is preferably equipped with acurrent non-injection region which blocks current injection into thedisordered portion. It is especially preferable that the length Ln ofthe current non-injection region is in the range of Lw≦Ln≦Lw+10 μm,setting the length of said disordered portion to Lw. Here, the termlength refers to the length along the direction of the resonator. It isalso preferable that the current non-injection region is embedded insaid semiconductor laser device and is a semiconductor layer having theconduction type opposite to that of the surrounding semiconductor layer.

It is also particularly preferable that the above-mentionedsemiconductor device is equipped with: n-type and p-type opticalwaveguide layers having a band gap energy larger than the band gapenergy of said active layer on both sides of the active layer,respectively; n-type and p-type cladding layers having a band gap energylarger than the band gap energy of said optical waveguide layer, in away to sandwich said active layer and said optical waveguide layer,respectively; and carrier block layers having a band gap energy largerthan that of each of said active layers and optical waveguide layers,between said active layers and said optical waveguide layers.

According to the present invention, upon fabrication of a semiconductordevice, especially of a semiconductor laser device provided with awindow structure, a protecting film was formed on the semiconductordevice surface corresponding to the portion not to be disordered, and adisordering-enhancing film on the semiconductor device surfacecorresponding to the portion to be disordered, in advance of thedisordering process, especially by the method utilizing thedecomposition reaction of the precursor represented by catalytic CVDmethod; so that adverse effects caused by the thermal treatment on theportion not to be disordered does not occur, and a semiconductor laserdevice of high output power and excellent long-term reliability can beprovided.

In the above-mentioned method utilizing the decomposition reaction ofthe precursor, the precursor is a compound containing nitrogen andsilicon or a mixture of nitrogen compounds and silicon compounds, sothat mixing of oxygen into the semiconductor crystal under the thermaltreatment for disordering does not take place, and a semiconductordevice of excellent long-term reliability can be provided.

Further, the current non-injection region is provided corresponding tothe disordered portion, so that current will not be injected into theportion with atomic vacancies formed by the disordering thermaltreatment, and the reliability of the crystal quality is improved. Inaddition, the non-light emitting recombination near the facet issuppressed, which is still more effective for prevention of COD, incombination with the window structure made by the disordering process.

In addition, in the semiconductor laser device of the window structuretype fabricated utilizing the above-mentioned method, carrier blocklayers having band gap energies larger than each of the band gapenergies of the active layer and the optical waveguide layer wereprovided between said active layers and said optical waveguide layers,so that, especially on the semiconductor laser device of an AlGaAssystem, an optical waveguide layer can be constituted by a layer of lowaluminum (Al) or a layer of GaAs. Because of this, the quality of there-grown interface accompanying with the fabrication of the currentnon-injection layer can be improved, and so the rise of the operationvoltage can be avoided and a semiconductor laser device of excellentlong-term reliability can be provided.

The inventor of the present invention have found out that by adjustingthe composition of the dielectric film, a difference can be introducedinto the ability of the dielectric film to absorb the constituent atomsfrom the compound semiconductor. Utilizing this principle, the inventorhas completed the present invention relating to a new method offabricating a semiconductor laser device. That is, the present inventionprovides a method of fabricating a quantum well semiconductor laserdevice having a window structure formed by disordering the quantum wellstructure. The method has a step of forming a protective film to preventdisordering on the portion where the quantum well structure is not to bedisordered, and a step of forming a disordering-enhancing film on theportion where the quantum well structure is to be disordered. Bychoosing the composition of the dielectric film to be formed in eachstep, respectively, it becomes possible to construct differentiallyeither a protective film or a disordering-enhancing film very easily andsurely. Therefore, according to the present invention, the process offabricating a semiconductor laser device having a region with disorderedquantum wells, such as a window structure for COD prevention forexample, will be simplified, and the yield will be improved.

BRIEF EXPLANATION OF DRAWINGS

FIGS. 1( a) and 1(b) are cross sections illustrating fabrication of anepitaxial wafer of a semiconductor laser device according to anembodiment of the present invention.

FIGS. 2( a) to 2(c) are cross sections illustrating processes offormation and patterning of dielectric films for forming windowstructures of a semiconductor laser device according to an embodiment ofthe present invention.

FIGS. 3( a) and 3(b) are longitudinal sections illustrating processesfor forming window structures of the semiconductor laser deviceaccording to an embodiment of the present invention.

FIGS. 4( a) and 4(b) are cross sections illustrating processes ofcleavage of and formation of a high reflective and a low reflectivefilms on the semiconductor laser device according to an embodiment ofthe present invention.

FIGS. 5( a) and 5(b) are cross sections seen from the facet side,illustrating fabrication of an epitaxial wafer of a semiconductor laserdevice according to an embodiment of the present invention.

FIG. 6 is a cross section illustrating a heating apparatus for formingthe window structure of the semiconductor laser device according to anembodiment of the present invention.

FIG. 7 is a schematic diagram illustrating the constitution of catalyticCVD method according to an embodiment of the present invention.

FIG. 8 is a graph showing the relationship between the refractive indexof a deposited SiN film (abscissa), and the amount of energy shift ofthe photoluminescence spectrum peak wavelength of wafers between beforeand after the heat treatment (meV, ordinate), in the case where a SiNfilm is formed on the compound semiconductor epitaxial wafer containingquantum well structure, using catalytic CVD method and plasma CVDmethod.

FIG. 9 is a cross section for illustrating another fabrication mode ofdielectric films, in the method of fabricating the semiconductor laserdevice according to an example of the present invention.

FIG. 10( a) is a schematic diagram illustrating distribution of the bandgap energy in SCH structure according to the present invention, and FIG.10( b) is a schematic diagram illustrating distribution of the band gapenergy in DCH structure.

FIG. 11 is a figure showing injection current dependence of opticaloutput power in a semiconductor laser device having window structuresfabricated by a method of fabrication according to an example of thepresent invention, and a semiconductor laser device not having windowstructures.

FIG. 12 is a schematic diagram illustrating a phenomenon during thermaltreatment for disordering in a semiconductor laser device havingconventional window structures.

THE DETAILED DESCRIPTION OF THE INVENTION

Hereafter, based on the drawings, a method of fabricating asemiconductor device according to an embodiment of the present inventionis explained.

[Fabricating Method]

FIGS. 1 to 5 are cross sections illustrating the method of fabricating asemiconductor device according to an embodiment of the presentinvention. This semiconductor device is a semiconductor laser device ofmultiple quantum well (MQW) structure emitting laser light of 0.98 μmband. FIGS. 1( a) and 1(b) are figures, cross sections along thedirection of a resonator, illustrating epitaxial wafer fabrication ofthe semiconductor laser device. FIGS. 5( a) and 5(b) show the crosssections of the epitaxial wafer fabrication in the directionperpendicular to the direction of the resonator. These figures are drawnby extracting the region that constitutes one semiconductor laser devicelater.

First, as shown in FIG. 1( a) and FIG. 4( a), an Al_(0.08)Ga_(0.92)Aslower cladding layer 2 with a thickness of 2.4 μm and a GaAs lowerwaveguide layer 3 with a thickness of 0.48 μm are grown on a GaAssemiconductor substrate 1 in this order. On the lower waveguide layer 3,an Al_(0.4)Ga_(0.6)As lower carrier block layer 4 c with a thickness of0.035 μm, a multiple quantum well active layer 4 b with two stackedIn_(0.14)Ga_(0.86)As quantum well layers each with a thickness of 0.01μm, and a Al_(0.4)Ga_(0.6)As upper carrier block layer 4 a with athickness of 0.035 μm are formed. The structure including these carrierblock layers 4 a and 4 c is a decoupled confinement heterostructure(DCH), explained later. After forming a GaAs upper waveguide layer 5 onthe upper carrier block layer 4 a partway, a stripe-shapedAl_(0.32)Ga_(0.68)As current non-injection layer 8 with a thickness of0.055 μm is selectively formed in the region stretching from theposition where a facet is to be made later to the position 20 μm apartin the center direction. The current non-injection layer 8 is alsoformed in the region on both sides of a stripe-shaped resonator in thelongitudinal direction, and thereby, the current injection region of themultiple quantum well active layer 4 b is defined in the stripe shape.Here, the conduction type of the current non-injection layer 8 is madeto be opposite to the conduction type of the upper cladding layer 6 tobe formed later.

Next, as shown in FIG. 1( b) and FIG. 4( b), the remaining upperwaveguide layer 5 is formed. The thickness of the upper waveguide layer5 containing the current non-injection layer 8 becomes 0.45 μm. Further,a Al_(0.32)Ga_(0.68)As upper cladding layer 6 with a thickness of 0.8 μmand a GaAs contact layer 9 with a thickness of 0.3 μm are formedsuccessively.

In addition, as for dopants doped in each layer, from the semiconductorsubstrate 1 to the contact layer 9, silicon is doped, for example, intothe semiconductor substrate 1, the lower cladding layer 2, the lowerwaveguide way 3, the current non-injection layer 8 and the lower carrierblock layer 4 c, in order to make their conduction types n-type; andzinc is doped, for example, into the upper waveguide layer 3, the uppercladding layer 6, the contact layer 9 and the upper carrier block layer4 a, in order to make their conduction types p-type. The multiplequantum well active layer 4 b is grown undoped.

FIGS. 2( a) to 2(c) are cross sections along the direction of theresonator, showing the process of forming a dielectric film on the uppersurface of the epitaxial wafer in advance of the thermal treatment fordisordering (mixed crystallization) in order to form the windowstructures.

First, as shown in FIG. 2( a), a SiN_(x1) protective film 10 having thethickness of 50 nm is deposited on the entire upper surface of thecontact layer 9, using catalytic CVD method. The SiN_(x1) protectivefilm 10 is a film with high compactness and small internal stress. Next,resist is applied to the upper surface of the SiN_(x1) protective film10, and patterning is performed on the resist by photolithography toform a resist mask 11 which covers the region not to be disordered, asmentioned later.

Then, after the SiN_(x1) protective film 10 in the region not coveredwith the resist mask 11 is etched by the reactive ion etching (RIE)using carbon tetrafluoride (CF₄), the resist mask 11 is removed withorganic solvent. Thereby, as shown in FIG. 2( b), the contact layer 9 isin an exposed state not being covered with SiN_(x1) protective film 10in the region to be disordered; and in the region other than that, it isin the state covered with the SiN_(x1) protective film 10.

As shown further in FIG. 2( c), using catalytic CVD method, a SiN_(x2)disordering-enhancing film 12 having the thickness of 25 nm is formed onthe entire upper surfaces of the exposed contact layer 9 and theSiN_(x1) protective film 10. The composition ratio x2 of the SiN_(x2)disordering-enhancing film 12 differs from the composition ratio x1 ofSiN_(x1) protective film 10. An explanation will be given later on thispoint.

FIGS. 3( a) and 3(b) are cross sections illustrating a process ofthermal treatment for disordering to form the window structure using adevice shown in FIG. 6.

An epitaxial wafer 13 equipped with a SiN_(x1) protective film 10 and aSiN_(x2) disordering-enhancing film 12 as described above is placed on amount 15 made of silicon carbide (SiC) installed in a quartz tray 14 asshown in FIG. 6. Then, in nitrogen (N₂) gas atmosphere, a short-timethermal treatment (RTA: Rapid Thermal Anneal) for 30 seconds at thetemperature of 930° C. with a lamp heater 16 arranged under the quartztray 14, is performed. By performing the RTA, gallium (Ga) atoms areabsorbed by the SiN_(x2) disordering-enhancing film 12 from the layerunder the SiN_(x2) disordering-enhancing film 12, and atomic vacanciesare generated near the surface of the contact layer 9. The atomicvacancies diffuse and reach especially the multiple quantum well activelayer 4 b, so that disordering takes place to form windows 28 as shownin FIG. 3( a). The quartz tray 14 has a lid 17 on it, and nitrogen gasis made to flow in and out the tray through the gas inlet 18 and the gasoutlet 19, with a flow rate of 2 L/min, for example.

Then, as shown in FIG. 3( b), the SiN_(x1) protective film 10 and theSiN_(x2) disordering-enhancing film 12 are removed by hydrofluoric acid.

Subsequently, the semiconductor laser device is completed according to aprocess illustrated in FIGS. 4( a) and 4(b). That is, after forming anupper electrode 21 and a lower electrode 22, the multilayer body iscleaved at the position near the center (position shown by dashed lineC) of the disordered regions in FIG. 4( a), and is separated as a laserbar consisting of plural semiconductor laser devices (the longitudinaldirection of the bar is perpendicular to the page space). Out of thecleaved facets of the laser bar thus separated, the light-emitting facetis coated by a low reflection film 23, and the light-reflecting facet iscoated by a high reflection film 24 as shown in FIG. 4( b). Finally, bybeing cut parallel to the page space, each semiconductor laser device inthe laser bar is separated in a chip shape, and the semiconductor laserdevice is completed.

The upper electrode 21 is a multiple metal layer formed on the contactlayer 9 consisting of, for example, titanium (Ti), platinum (Pt), andgold (Au) layers sequentially, and the lower electrode 22 is constitutedwith a structure formed at the undersurface of the semiconductorsubstrate 1, for example, by gold, germanium, nickel (AuGeNi) alloy orwith a gold layer on it in addition.

[Explanation of Dielectric Film Fabrication by Catalytic CVD Method]

The SiN_(x1) protective film 10 and the SiN_(x2) disordering-enhancingfilm 12 which are formed by the processes illustrated in FIGS. 2( a) to2(c) are formed with catalytic CVD method, as described above. FIG. 7 isa diagram showing the outlined constitution of a catalytic CVDapparatus.

In the catalytic CVD apparatus shown in FIG. 7, a vacuum pump 37 isconnected to a chamber 31 through a pressure adjustment valve 38. In thechamber 31, a substrate holder 35 having a substrate heater 36 isprovided. On the substrate holder 35, an epitaxial wafer 34, on which aprotective film 10 and a disordering-enhancing film 12 are to bedeposited, is placed. In the chamber 31, a tungsten wire 33 for heatingis provided above the epitaxial wafer 34, and a showerhead 32 is furtherprovided above the wire.

When the protective film 10 or the disordering-enhancing film 12 isdeposited using the catalytic CVD apparatus having such a constitution,the substrate heater 36 is pre-heated at about 200° C. to about 300° C.before the epitaxial wafer 34 is placed on the substrate holder 35.Then, the vacuum pump 37 is operated after placing the epitaxial wafer34, and the inside of the chamber 31 is decompressed to a predeterminedpressure, for example, about 1×10⁻⁴ Pa.

Ammonia (NH₃) is introduced into the chamber 31 via the shower head 32at a predetermined flow rate f_(NH3), and the tungsten wire 33 iselectrified at the same time so as to maintain the temperature of thetungsten wire 33 at 1650° C. Silane (SiH₄) is introduced via the showerhead 32 at a predetermined flow rate of f_(SiH4) to keep the pressure inthe chamber 31 at 4.0 Pa.

The molecules of SiH₄ and NH₃ introduced in the chamber 31 contact thetungsten wire 33 heated to about 1600° C. to about 2000° C., and,catalyzed thereby, decompose and are activated into SiH_(y) and NH_(z),which are thermally desorbed and react on the wafer 34 heated with thesubstrate-heater 36, and deposit on the wafer in the form of SiN_(x).

By the way, both SiN_(x1) protective film 10 and SiN_(x2)disordering-enhancing film 12 are SiN_(x) formed by above-describedcatalytic CVD method, and whether the SiN_(x) may function as theprotective film 10 or as the disordering-enhancing film 12 is determinedby its composition. That is, by setting up appropriately theabove-described source gas flow rates f_(NH3) and f_(SiH4), it ispossible to construct differentially either the protective film 10 orthe disordering-enhancing film 12 using catalytic CVD method.

For example, in the case of fabricating the semiconductor laser of 980nm band, under the condition that the film is formed by setting the gaspressure at the time of film formation, that is, the pressure in thechamber 31, to be 4.0 Pa, the present inventor has found that, separatedby a boundary where the deposited SiN_(x) has a composition at which therefractive index is nearly 1.96, the deposited film functionsdifferently: that is, the film with the composition of more Si than that(the refractive-index>1.96) has higher atomic density and functions asthe protective film 10; and the film with the composition of less Si(the refractive-index<1.96) has lower atomic density and functions asthe disordering-enhancing film 12.

For example, FIG. 8 shows measurements of the following experiment.SiN_(x) films of various composition were deposited on the surface ofthe epitaxial wafer for fabricating the semiconductor laser explained inthe embodiment, by keeping the flow rate of ammonia constant andchanging the flow rate of silane, then thermal treatment at 980° C. for30 seconds was performed, and the disordering degree of the quantum wellactive layer was measured. The disordering degree is given by the energyshift (meV), which is calculated from the magnitudes of the shift of thephotoluminescence spectrum peak wavelengths at the room temperatureobserved between the wavelengths before and after the thermal treatment.In FIG. 8, the plots given by black circle marks show the measurementswhen the SiN films were formed with catalytic CVD method, and the plotsgiven by ∘ marks show the measurements wherein the SiN films were formedwith plasma CVD method (PECVD: Plasma Enhanced CVD) as a comparativeexample. The conditions of the film formation used in each method forfilm formation are as follows. When the flow rate of silane is increasedin FIG. 8, the refractive index of the film becomes larger.

(Film Formation Conditions of the Sin Film by Catalytic CVD Method)

Film thickness of the deposited SiN film: 50 nm,

Gas pressure (Pressure in the chamber): 4.0 Pa,

Substrate temperature: 250° C.,

Tungsten wire temperature: 1650° C.,

Flow rate of ammonia: 0.2 L/min,

Flow rate of silane: 0.001 to 0.003 L/min.

(Film Formation Conditions of the Sin Film by Plasma CVD Method)

Film thickness of the deposited SiN film: 50 nm,

RF power: 190 mW,

Gas pressure (Pressure in the chamber): 50 Pa,

Substrate temperature: 250° C.,

Flow rate of nitrogen: 0.28 L/min,

Flow rate of silane: 0.004 to 0.008 L/min,

In FIG. 8, the film composition in the case of plasma CVD method waschanged by altering the flow rate of supplied silane.

As seen in FIG. 8, in the case where the SiN film is formed withcatalytic CVD method under the above-described film formationconditions, the magnitude of energy shift seen between after and beforethe thermal treatment is small when the refractive index of thedeposited film is larger than about 1.96, and the magnitude of energyshift is large when it is smaller than 1.96. This means that thedeposited film functions as the protective film 10 when its refractiveindex is larger than about 1.96, and functions as thedisordering-enhancing film 12 when it is smaller than 1.96.

Based on this finding, in the case of forming the SiN_(x1) protectivefilm 10 in the present embodiment, the flow rates were set tof_(NH3)=0.2 L/min and f_(SiH4)=2.5×10⁻³ L/min, and in the case offorming the SiN_(x2) disordering-enhancing film 12, the flow rates wereset to f_(NH3)=0.2 L/min and f_(SiH4)=2.0×10⁻³ L/min. The pressure inthe chamber was 4.0 Pa when either of dielectric films was formed. Therefractive indices of the protective film and the disordering-enhancingfilm measured under these conditions were 2.02 and 1.94, respectively.

The relationship between the composition of formed SiN_(x) and the flowrate of the source gas may be different depending on every catalytic CVDapparatus or on every film formation condition (the gas pressure, thesubstrate temperature, the tungsten wire temperature, and the like atthe time of film formation), so that it is desirable to check thecomposition of SiN_(x) by such as the refractive-index measurement forevery catalytic CVD apparatus or for every film formation condition.

For example, when the SiN film is formed in the same way as above bysetting the gas pressure at the time of film formation to 2.0 Pa, theinventor has found that separated by a boundary at which the compositionhas the refractive index of nearly 2.07, the deposited SiN film with acomposition of more Si (the refractive index>2.07) functions as a highdensity protective film, and the SiN film with the composition of lessSi (refractive index<2.07) functions as a low densitydisordering-enhancing film. In this way, by determining the boundarycomposition for every catalytic CVD apparatus or the film formationconditions with regards to whether the obtained film functions as aprotection film or as a disordering-enhancing film, and by adjusting theflow rate of the source gas based on this, either the protection film orthe disordering-enhancing film can be fabricated differentially.

Although the reason why the deposited film is determined to function asa protective film or as a disordering-enhancing film, separated by thepredetermined boundary flow rate of source gas which is dependent on thefilm formation conditions, has not yet been completely elucidated, theinventor thinks as follows with regards to this. That is, in the casewhere the dielectric films (SiN_(x) films) with various compositions areformed with catalytic CVD method under various flow rate of silane, ifthe flow rate of silane is large, the film takes up many Si atoms andbecomes a high density film, and if the flow rate of silane is small, itbecomes a low density film. The dielectric film with low density isthought to have large skeletal atom intervals and so when it is formedon a semiconductor crystal and heat treated, Ga atoms belonging to theIII group are easily absorbed into the dielectric film from thesemiconductor crystal. That is, atomic vacancies are easy to be formedin the semiconductor crystal by a lack of atoms.

In contrast to this, the skeletal atom intervals are thought to benarrow in the dielectric film with high density. Therefore, when thedielectric film is formed on the semiconductor crystal and heat treated,Ga atoms are rarely absorbed into the dielectric film from thesemiconductor crystal. That is, atomic vacancies are difficult to beformed in the semiconductor crystal.

Due to such differences in the property, the dielectric film with highdensity functions as the protective film for disordering of the quantumwell, and the film with low density functions as the enhancing film.

Although measurement of the magnitude of the film density is generallydifficult, it can be discriminated by measuring the refractive index. Inthe present example, the inventor has found out that it is possible todetermine that, as shown in FIG. 8, when the refractive index is largerthan the predetermined value decided depending on the film formationconditions, the film functions as a protection film, and when it issmaller than said predetermined value, the film functions as adisordering-enhancing film, respectively.

Further, when the composition ratio of Si contained in the dielectricfilm is larger than the stoichiometric composition ratio of Si of saiddielectric film, the film functions as a large density protective film,and when the composition ratio of Si is smaller than the stoichiometriccomposition ratio of Si of said dielectric film, it functions as a lowdensity disordering-enhancing film.

Next, paying attention on the quantity of the hydrogen atoms taken intothe dielectric film, a film containing more hydrogen atoms in it haslarger skeletal atom intervals, and so when it is formed on asemiconductor crystal and heat treated, Ga atoms are easily absorbedinto the dielectric film from the semiconductor crystal. That is, atomicvacancies are easily produced in the semiconductor crystal. In contrastto this, a film containing small amount of hydrogen has narrow skeletalatom intervals, and so when it is formed on a semiconductor crystal andheat treated, Ga atoms are rarely absorbed into the dielectric film fromthe semiconductor crystal. That is, atomic vacancies are rarely producedin the semiconductor crystal. Therefore, a dielectric film containinglow hydrogen concentration in the film functions as a protective filmand the one containing high hydrogen concentration functions as anenhancing film.

In addition, as is seen in FIG. 8, the magnitude of energy shift betweenbefore and after the thermal treatment changes more steeply around astandard value determined according to the film formation conditions inthe case where the film is formed using catalytic CVD method as comparedwith the case where it is formed using plasma CVD method. Theconceivable reason is that a compact film is easily formed with thecatalytic CVD method and so this method is suitable for fabricating aprotective film and a disordering-enhancing film differentially bymaking use of a film density difference. However, by using filmformation conditions to acquire a compact film with some certainty, thepresent invention may be applicable by adopting film formation methodsother than the catalytic CVD method.

Since the SiN_(x1) protective film 10 is formed with the catalytic CVDmethod on the region where the window is not formed, the SiN_(x1)protective film 10 is compact and with low stress, and so desorption ofatoms such as As from the semiconductor surface during the thermaltreatment for disordering can be fully prevented. Therefore, accordingto the present invention, since such a problem that the pits produced bydesorption of As atoms rough the surface of the contact layer 9 does notarise, a favorable film contact to the upper electrode 21 is assured.Further, since the semiconductor is free from the problem that pits makedislocation defects which spread to the active layer during the laseroperation, the semiconductor laser excellent in long-term reliabilitycan be obtained.

Main advantages obtained by using the SiN_(x) film produced by catalyticCVD method as the disordering-enhancing film, instead of usingconventional SiO₂, are explained below, taking into account theexperimental results.

(Advantages of Using the SiN_(x) Disordering-Enhancing Film Fabricatedby Catalytic CVD Method: 1)

Because SiN_(x) is used instead of conventional SiO₂ as thedisordering-enhancing film in the present embodiment, mixing of oxygenatoms into the semiconductor crystal may not take place. That is, fromthe SiO₂ film conventionally formed on the surface of a semiconductorcrystal, oxygen atoms diffuse into the semiconductor multilayer to makecrystal defect and cause lowering of the long-term reliability. However,since SiN_(x) does not contain oxygen essentially, there are fewproblems caused by oxygen.

(Advantage of Using the SiN_(x) Fabricated by Catalytic CVD Method: 2)

In order to pattern the SiN_(x1) protective film 10 only on the regionnot to be disordered in the present embodiment, the contact layer 9 atthe end part of the semiconductor laser is exposed by etching theSiN_(x1) protective film 10 with the reactive ion etching using CF₄(FIG. 2( b)). Then SiN_(x2) disordering-enhancing film 12 is depositedon it, and the interface between the contact layer 9 and the film 12 isvery favorable since the catalytic CVD method is used for depositing theSiN_(x2) disordering-enhancing film 12. Conceivably, it may be becausein the catalytic CVD method, the surface of the contact layer 9 isetched by hydrogen radicals, which has a cleaning effect. On the otherhand, when the disordering-enhancing film is formed on the contact layer9 which had received fluoride etching process by using frequently usedconventional methods, such as the plasma CVD method and the EB vapordeposition, pit-like roughness may sometimes occur on the surface of thecontact layer 9 under the SiN_(x2) disordering-enhancing film 12.

The following experiments were conducted in order to confirm this. Thefollowing two kinds of samples A and B were fabricated by using theepitaxial wafer having the same laser structure as one shown in thepresent embodiment. On the semiconductor multilayer of the sample A, RIEprocessing using CF₄ was performed and then SiO₂ film was formed by EBvapor-depositing method. The SiO₂ film was formed to the thickness of 20nm by setting the substrate temperature to 180° C. On the semiconductormultilayer of the sample B, RIE processing using CF₄ was performed andthen a SiN_(x) film was formed by catalytic CVD method. The SiN_(x) filmwas formed with the thickness of 50 nm, setting the flow rate of silane,f_(SiH4)=2×10⁻³ L/min. Numbers of the pits generated on thesemiconductor multilayer surface were measured on the samples A and B.The sample A had 3000 pits/cm², and the sample B had 500 pits/cm² orless. In fabricating the sample B, the number of pits did not increaseeven if the flow rate of SiH₄ was changed.

In this way, the number of pits produced on the surface of the compoundsemiconductor multilayer below the dielectric film can be suppressed tobe low by forming the dielectric film with catalytic CVD method.Thereby, reliability of a semiconductor laser device is expected to besecured.

As the formation order of the dielectric films, the present embodimenthas explained the case where after forming a protective film on thesemiconductor laser surface in the region other than the region to bedisordered, the disordering-enhancing film covering at least the regionto be disordered is formed. Forming the low densitydisordering-enhancing film on the high-density protective film in thisway has an advantage that the gas absorbed in the film during theprotective film formation is efficiently released to outside through thelow density disordering-enhancing film during the thermal treatment fordisordering.

However, the formation order of the dielectric film is not restricted tothe above order, but may also be reversed. That is, as shown in FIG. 9,when the disordering-enhancing film is formed in advance, and next theprotective film is formed so as to cover the enhancing film from aboveto cover the region to be disordered, impurities such as Ga atomsexisting in the atmosphere in the thermal treatment furnace do notdissolve into the low density disordering-enhancing film from theexterior to diffuse during the thermal treatment. It is thusadvantageous because the change in the amount of Ga absorbed by thedisordering-enhancing film from the semiconductor multilayer during thethermal treatment is suppressed and the function as thedisordering-enhancing film is stabilized

Although SiN_(x) was used as the protective film and thedisordering-enhancing film in this embodiment, it is needless to saythat other kinds of dielectric films may be adopted, as long as they canabsorb the constituent atoms in the semiconductor crystal to generateatomic vacancies in said semiconductor crystal, and the density of thedeposited film is controllable by the film formation conditions. Thedeposition method of these dielectric films is also not restricted tothe catalytic CVD method, and as long as the film formation conditionscapable of controlling the densities of the deposited films are used,plasma CVD method, EB vapor-depositing method, spin coat method and thelike can be used, for example.

[Current Non-Injection Structure]

Next, as shown in FIG. 4( b), a current non-injection structure formedby the fabricating method according to one aspect of the presentinvention has a layer of length Ln with the conduction type opposite tothat of the upper cladding layer 6, near the facet in the upper claddinglayer 6. In this current non-injection structure, the length Lw of thewindow is 10 μm and the length Ln of the current non-injection layer 8is 20 μm which is longer than the length Lw of the window. Due to theexistence of current non-injection layer 8, the current supplied to thesemiconductor laser is not injected into the region where the atomicvacancies were introduced by the thermal treatment for disordering, andso degradation of the crystal quality is prevented and reliability ofthe semiconductor laser device is improved. The non-light-emittingrecombination near the facet is suppressed, which is still moreeffective for prevention of COD, jointly with the window structureformed by disordering. If Ln is made longer than Lw+10 μm, enoughcurrent may not be injected into the active layer region. For thisreason, the length Ln of the current non-injection layer 8 is preferablyless than Lw+10 μm, setting the length of the disordered portion (thewindow) measured from the end of the semiconductor laser device to Lw.The relation, Ln<Lw may also be possible. Here, the length of Ln and Lware the lengths along the longitudinal direction of the resonator.

The current non-injection layer 8 is also formed consecutively in theregions on both sides of the stripe-shaped resonator in the longitudinaldirection, in order to serve, in addition, as a low refractive indexlayer for confining light in the transverse direction. Therefore, boththe transversal confining structure of the waveguide mode and thecurrent non-injection structure can be fabricated at once by one maskpatterning for forming the current non-injection layer 8.

Such a current non-injection structure is formed as shown in FIGS. 1 and5. After forming the upper optical waveguide layer 5 partway above theupper carrier block layer 4 a formed on the multiple quantum well activelayer 4 b, the stripe-shaped semiconductor layer (the currentnon-injection layer) 8 is selectively deposited in the region stretchingfrom the position which is destined for a facet of the semiconductorlaser later to the position which is apart by the length Ln in thecenter direction (see FIG. 1( a)), and in the region on both sides ofthe stripe-shaped resonator in the longitudinal direction (see FIG. 5(a)); and then, the remaining upper optical waveguide layer 5 is formedto bury said semiconductor layer 8. Here, the conductive type of thecurrent non-injection layer 8 is made opposite to that of the upperwaveguide layer 5 in which it is embedded.

Although a semiconductor layer 8 having a conduction type opposite tothe upper waveguide layer 5 is embedded in the upper optical waveguidelayer 5 in the above explanation, the current non-injection structuremay be formed by embedding a semiconductor layer having a conductiontype opposite to the lower waveguide layer 3 in the lower waveguidelayer 3, or may be formed by embedding the semiconductor layer havingthe conduction type opposite to each conduction type in both the upperwaveguide layer 5 and the lower waveguide layer 3.

[DCH Structure]

The DCH structure formed by the fabricating method according to oneaspect of the present invention has carrier block layers in thewaveguide region. On the other hand, as a high-output semiconductorlaser device, the separate confinement heterostructure (SCH) has beenoften used conventionally. In FIGS. 10( a) and 10(b), the band gapenergy distributions (left side ordinate) and the refractive indexdistributions (right side ordinate) of both structures are shown. FIG.10( a) shows the SCH structure having optical waveguide layers 3′ and 5′on both sides of an active layer 4′. FIG. 10( b) shows the DCH structureadopted by the semiconductor laser device of the present embodiment. Thedistribution-shapes of the band gap energy and the refractive index ofeach layer inside the quantum well structures of the active layers 4 and4′ are omitted.

The guide mode of the laser light emitted from the DCH structure iscloser to Gauss-type compared with the SCH structure and the light-leakto the cladding layer is small. So that, when the lasers are designed tohave the same oscillation wavelength and the same radiation angle, thefilm thickness (L2 in FIG. 10( b)) of the whole laser structure of theDCH-structured laser can be thinner compared with the whole filmthickness (L1 in FIG. 10( a)) of the SCH-structured laser. Therefore, inthe semiconductor laser device having a window structure formed as aresult of disordering of the multiple quantum well layers by diffusionof atomic vacancies, the diffusion length of atomic vacancies requiredfor disordering can be shortened by adopting the DCH structure. For thisreason, the thermal treatment for disordering can be performed at alower temperature, and the damage given by the thermal treatment fordisordering to the laser crystallinity can be suppressed to a minimum.

In the SCH structure, in order to confine the carriers efficiently inthe active layer, Al composition ratio of the optical waveguide layer 3′has had to be high to some extent. On the other hand, in the DCHstructure, since the carrier block layers 4 a and 4 c assume the role toconfine the carriers, the optical waveguide layer 3 does not have to beof high Al composition ratio, and can be constituted from GaAs. If theoptical waveguide layer is constituted from GaAs, an accumulation ofoxygen to the re-grown interface, which tends to take place in theAlGaAs layer of high Al composition ratio, will be suppressed. Thus,formation of a potential barrier at the re-grown interface issuppressed, and rise in the operating voltage can be avoided. Since thenon-light-emitting recombination is suppressed by the suppression ofoxygen accumulation at the re-grown interface, the semiconductor laserdevice has excellent long-term reliability.

Such a DCH structure is formed, as shown in FIGS. 1 and 5, by depositingthe upper carrier block layer 4 a and the lower carrier block layer 4 ceach having band gap energies larger than each band gap energy of theoptical waveguide layers 5 and 3, between the multiple quantum wellactive layer 4 b and the upper optical waveguide layer 5, and betweenthe multiple quantum well active layer 4 b and the lower opticalwaveguide layer 3, respectively.

Example

On a GaAs substrate of 2 inches in diameter, a semiconductor multilayerstructure was formed as shown in FIG. 1( b). The multilayer structureconsists of a n-Al_(0.08)Ga_(0.92)As lower cladding layer 2 with athickness of 2.4 μm, a n-GaAs lower waveguide layer 3 with a thicknessof 0.48 μm, a n-Al_(0.4)Ga_(0.6)As lower carrier block layer 4 c with athickness of 0.035 μm, a multiple quantum well active layer 4 b with twostacked In_(0.14)Ga_(0.86)As quantum well layers each with a thicknessof 0.01 μm, a p-Al_(0.4)Ga_(0.6)As upper carrier block layer 4 a with athickness of 0.035 μm, an upper waveguide layer 5 with a thickness of0.45 μm, a p-Al_(0.32)Ga_(0.68)As upper cladding layer 6 with athickness of 0.8 μm, and a p-GaAs contact layer 9 with a thickness of0.3 μm grown in this order on a n-GaAs substrate 1. A stripe-shapedn-Al_(0.32)Ga_(0.68)As layer 8 with a thickness of 0.055 μm is embeddedat the predetermined intervals in the upper waveguide layer 5. Thephotoluminescence peak wavelength measured on the semiconductorsubstrate having such a multilayer structure corresponded to 1.276 eV interms of the band gap energy.

Next, as shown in FIG. 2( a), using catalytic CVD method, the SiN_(x1)protective film 10 with a thickness of 50 nm was formed on the entireupper surface of the contact layer 9. Film formation by the catalyticCVD method was performed, setting the pressure in the chamber to 4.0 Pa,temperature of the substrate at 250° C. and temperature of the tungstenwire at 1650° C., and by supplying ammonia and silane as the sourcegases with the flow rate of 0.2 L/min and 0.0025 L/min, respectively.The refractive index of the deposited SiN_(x1) protective film 10measured with an ellipsometer was 2.02.

Thereafter, resist patterning was performed on the upper surface of theSiN_(x1) protective film 10 using photolithography to form a resist mask11 covering the semiconductor except for the region near the facets. TheSiN_(x1) protective film 10 was etched using this resist mask 11 by thereactive ion etching (RIE) using carbon tetrafluoride (CF₄), and thenthe resist mask 11 was removed by an organic solvent (FIG. 2( b)).Thereby, a part of contact layer 9 was exposed.

On the entire upper surface of the exposed contact layer 9 and theSiN_(x1) protective film 10, the SiN_(x2) disordering-enhancing film 12with a thickness of 25 nm was formed using catalytic CVD method (FIG. 2(c)). The film formation by catalytic CVD method was performed, bysetting the pressure in a chamber to 4.0 Pa, the temperature of thesubstrate at 250° C. and the temperature of the tungsten wire at 1650°C., and supplying ammonia and silane as the source gases with the flowrate of 0.2 L/min and 0.002 L/min, respectively. The refractive index ofthe deposited SiN_(x)2 disordering-enhancing film 12 measured with anellipsometer was 1.94.

Next, as shown in FIG. 6, the semiconductor substrate was placed on amount 15 made of silicon carbide (SiC) installed in a quartz tray 14,and was heated in nitrogen (N₂) gas atmosphere for 30 seconds at thetemperature of 930° C. with a lamp heater 16 arranged under the quartztray 14. Then the SiN_(x1) protective film 10 and the SiN_(x2)disordering-enhancing film 12 were removed with hydrofluoric acid (FIG.3( b)). Then, the photoluminescence peak wavelengths were measured atthe portions where the SiN_(x1) protective film 10 and the SiN_(x2)disordering-enhancing film 12 had been formed, and were compared withthose measured before the thermal treatment. At the portion where theSiN_(x1) protective film 10 had been formed, a shift(wavelength-shortening) of about 5 meV or less in terms of band gapenergy was observed, while at the portion where the SiN_(x2)disordering-enhancing film 12 had been formed, a shift of about 35 meVwas observed.

Thereafter, the GaAs substrate was cleaved near the center of then-Al_(0.32)Ga_(0.68)As layer 8 embedded in a shape of stripe (FIG. 4(a)), to be plural laser bars. And the cleaved surface on the sidedestined for the light emitting facet was coated by a low reflectionfilm 23, and the cleaved surface on the opposite side was coated by ahigh reflection film 24 (FIG. 4( b)). Finally, each laser bar wasdivided with the prescribed interval, and individual semiconductor laserdevices were obtained.

On the semiconductor laser device fabricated in this way, the currentoptical output power characteristic (the injection current dependence ofthe optical output power) was measured. This is shown in FIG. 11. As acomparative example, the current optical output power characteristic wasalso measured on the semiconductor laser device without the windowstructure. As shown by the curves L2 in FIG. 11, in the semiconductorlaser device without the window structure, the optical output powersuddenly became zero due to COD when the injection current reached acertain level. On the other hand, in the semiconductor laser deviceproduced with the fabricating method of the present example, COD did notoccur but only reduction of the optical output power by heat saturationwas observed.

Moreover, this semiconductor laser device has such features that, in thefabricating process, removal of As atoms from the semiconductor surfacewith the thermal treatment for disordering is stopped, and nocontamination of oxygen atoms occurs during the thermal treatment fordisordering; and that the current non-injection structure and the DCHstructure are adopted, so that it shows a long-term excellentreliability.

Although in the above-described explanation, the case where thefabricating method according to the present invention was applied to thesemiconductor laser device of 0.98 μm band was explained, thefabricating method of the present invention is applicable also to thesemiconductor laser devices of other wavelength bands. It is needless tosay that the fabricating method of the present invention can be appliedto both semiconductor laser devices generating a transverse single modeoscillation and generating a transverse multiple mode oscillation. Inaddition, although the above explanation was about the semiconductorlaser device having a single light emitting stripe, it is needless tosay that the present invention can be applied to the array laserarranged by two or more light emitting stripes, either.

The explanation was done on the semiconductor laser having a multiplequantum well layer in the above explanation. However, when asemiconductor laser with a single quantum well layer is formed, thewindow structure may be also formed using the fabricating method of thepresent invention to prevent COD. Moreover, the fabricating method ofthe present invention can be used not only to form the window structureson the semiconductor device for preventing COD, but also can be usedmore generally to broaden the energy band gap of a specific portion of acompound semiconductor multilayer of a semiconductor device. Forexample, if the disordering-enhancing film is formed on the regions ofthe surface of the contact layer 9 which are above the regions locatingon both sides of the current injection region of the active layer 4 b,and is heat treated, said regions locating on both sides of the activelayer 4 b will be disordered and the refractive index thereof willbecome small. Thus, the distribution of the refractive indices in thetransverse direction consisting of the disordered region and the activelayer can function to confine the light in the transverse direction.

1. A method of fabricating a semiconductor device, the method comprisingthe steps of: forming a predetermined semiconductor multilayer whichincludes at least a quantum well active layer on a semiconductorsubstrate; forming a disorder-suppressing film on a first portion of thesurface of said semiconductor multilayer; forming a disorder-enhancingfilm on a second portion of the surface of said semiconductormultilayer, said disorder-enhancing film being made of the samedielectric material as said disorder-suppressing film and having a lowerrefractive index than said disorder-suppressing film in order thatcatastrophic optical damage (COD) to the laser type semiconductor deviceis reduced; performing thermal treatment of a multilayer body containingsaid semiconductor multilayer, said disorder-suppressing film and saiddisorder-enhancing film, thus disordering said quantum well layer undersaid disorder-enhancing film so as to have an increased band gap energyhigher than that of said quantum well layer under saiddisorder-suppressing film; and cleaving said multilayer body at nearlythe central part of said second portion.
 2. The method of fabricatingthe semiconductor device according to claim 1, wherein the refractiveindex of said disorder-suppressing film is larger than a predeterminedvalue determined depending on the film formation conditions of saiddisorder-suppressing film and said disorder-enhancing film, and therefractive index of said disorder-enhancing film is smaller than saidpredetermined value.
 3. The method of fabricating the semiconductordevice according to claim 1, wherein said disorder-suppressing film andsaid disorder-enhancing film are silicon nitride films.
 4. The method offabricating the semiconductor device according to claim 1, wherein saiddisorder-suppressing film is formed by the steps of: arranging a heatsource in a chamber on a path through which a first precursor of saiddisorder-suppressing film passes to cause a decomposition reaction ofsaid first precursor in the presence of said heat source, and exposingsaid first portion of said semiconductor device in said chamber; andsaid disorder-enhancing film is formed by the steps of: arranging a heatsource in said chamber on a path through which a second precursor ofsaid disorder-enhancing film passes to cause a decomposition reaction ofsaid second precursor in the presence of said heat source, and exposingsaid second portion of said semiconductor device in said chamber.
 5. Themethod of fabricating the semiconductor device according to claim 1,wherein said disorder-suppressing film is formed by the steps of:arranging a heat source in a chamber on a path through which a firstprecursor of said disorder-suppressing film passes to cause adecomposition reaction of said first precursor in the presence of saidheat source, and exposing said first portion of said semiconductordevice in said chamber; and said disorder-enhancing film is formed bythe steps of: arranging a heat source in said chamber on a path throughwhich a second precursor of said disorder-enhancing film passes to causea decomposition reaction of said second precursor in the presence ofsaid heat source, and exposing said second portion of said semiconductordevice in said chamber; and said first and second precursors arecompounds containing nitrogen and silicon, or mixtures of nitrogencompounds and silicon compounds.
 6. The method of fabricating thesemiconductor device according to claim 4, wherein said first precursorand said second precursor contain silane and ammonia, and the silanecontent in said first precursor is larger than the silane content insaid second precursor.
 7. The method of fabricating the semiconductordevice according to claim 4, wherein said disorder-suppressing film anddisorder-enhancing film are respectively formed by the step of causingdecomposition reactions of said first precursor and said secondprecursor with catalytic CVD method.
 8. The method of fabricating thesemiconductor device according to claim 1, wherein said predeterminedsemiconductor multilayer is formed by the steps of: forming an opticalwaveguide layer, at least on one side of said quantum well layer in thelayer forming direction, and embedding a semiconductor layer with aconduction type opposite to the conduction type of said opticalwaveguide layer into said optical waveguide layer at the portion beneathsaid second portion.
 9. The method of fabricating the semiconductordevice according to claim 1, wherein said predetermined semiconductormultilayer is formed by the steps of: forming optical waveguide layershaving band gap energies larger than the band gap energy of said quantumwell layer on both sides of said quantum well layer in the layer formingdirection, respectively; forming cladding layers having band gapenergies larger than the band gap energies of said optical waveguidelayers on both sides in the layer forming direction of the multilayerstructure comprising said quantum well layer and said optical waveguidelayers, respectively; and forming carrier blocking layers having bandgap energies larger than each band gap energy of said optical waveguidelayers between said quantum well layer and said optical waveguidelayers.
 10. The method of fabricating the semiconductor device accordingto claim 1, wherein said predetermined semiconductor multilayer isformed by the step of forming either a single or a multiple quantum wellstructure.